The field of biomedical optics has witnessed rapid growth over the last decade. The advent of new technology has permitted the development of time point spread function (TPSF)-based approaches which enable measurements to decouple the attenuation coefficient into absorption and scattering coefficients. This is beyond the fundamental limit of continuous wave approaches whose measurements only yield the attenuation coefficient.
Measurements of absorption of chromophores within tissues is a very valuable source of information since spectroscopic analysis of the tissue absorption spectrum permits the chromophore concentrations to be determined. Analysis of the chromophore concentrations can in turn yield physiological information and therefore provide a more medically useful image. TPSF based approaches are well suited for extracting absorption coefficients but the approach relies on expensive and complex hardware and software in order to perform the TPSF-based measurements.
It is recognized that CW techniques are less expensive and simpler than the TPSF-based approach. However, it is well-known that the CW approach can only measure the attenuation coefficient and cannot decouple this into the absorption and scattering coefficients. This non-uniqueness problem for CW has been demonstrated mathematically by Arridge et al. (Optics Letters Vol. 23, No. 11, 1998 pp 882-884). The inability of CW to measure the absorption coefficient and only provide the attenuation coefficient is considered to be a contributory reason for the clinical poor performance of previous CW breast imaging attempts.
One approach to circumvent the non-uniqueness of CW is to simply assume that the scattering coefficient is known a priori. However, such assumptions for the scattering coefficient are simply best “guesstimates” and its implementation has not been demonstrated to be clinically viable. Particularly since the scattering coefficient for even very similar tissues (such two breasts of the same patient) can vary widely and its value is not homogenous for a given tissue.
Tromberg et al (Proc. SPIE Vol. 4250 pp 437-442), have developed a TPSF-based approach in the frequency domain in order to measure the absorption and scattering coefficients of a selected type of tissue providing a homogenous medium at a few near infrared (NIR) wavelengths. By assuming a law for scattering coefficient versus wavelength, they derive the scattering coefficient over a wide wavelength range by fitting this law to the scattering coefficients at the few wavelengths measured by the TPSF-based approach. This is different than actually measuring the scattering coefficient over the complete wavelength range and therefore avoids the long acquisition time of TPSF-based information at all wavelengths of interest. A CW approach is then used to derive the attenuation coefficient over this wavelength range.
By using the attenuation coefficient from the CW approach, a few measured scattering coefficients from the TPSF-based approach, and many derived values for scattering coefficient from the scatter-law, values for the absorption coefficient over the complete wavelength range can be calculated to fully characterize the homogenous medium, namely to obtain its absorption coefficient spectrum and its scattering coefficient spectrum that follows a scatter law. This extra absorption coefficient information is considered to yield better estimates of the tissue chromophore concentrations than that provided by the few absorption coefficients measured by the TPSF-based approach alone.
Tromberg et al. have only used this approach for global/localized spectroscopy where the tissue region of interest is assumed to be homogeneous. Furthermore, their approach still requires the acquisition of TPSF based optical measurements in addition to the continuous wave measurements.
Thus, there is a need for an improved method for estimating concentrations of chromophores within tissue.